plugins: Add more check for allowed raw caps.

The gst_vaapi_plugin_base_get_allowed_raw_caps is used for both sink
pad and src pad, which cause some bugs. For sink pad, we need to verify
vaPutImage() while for the src pad we need to verify vaGetImage().
For vaapidecoderXXX kind of plugins, the case is more complex. We need
to verify whether the decoded result(in some surface, NV12 format most
of the time) can be vaGetImage to some raw image format. Add more check
to fix all these problems.
gstreamer/gstreamer-vaapi#123
Signed-off-by: He Junyan <junyan.he@hotmail.com>